Wholly aromatic polyesters having various structures have been proposed and it is known that they have desirable properties including excellent heat resistance and mechanical strength. However, technology on films of wholly aromatic polyester films having such desirable properties, in particular on films of liquid crystalline wholly aromatic polyester, is scarcely known. Known in the art are only those which are not transparent and those composed of special polyesters, involving various problems discussed below.
Japanese Patent Publication No. 47-47870 states that wholly aromatic polyesters disclosed therein may be made into film. But it makes no mention of properties of any film and of any process of preparing films.
Japanese Patent Publication No. 53-45334 discloses a film obtained by frame-spraying particulate wholly aromatic polyester on a heated steel plate. The film so prepared is, however, poor in transparency because a melt of said particulate wholly aromatic polyester is anisotropic.
Japanese Patent Laid-open Publication No. 54-77691 discloses that a melt of wholly aromatic polyesters disclosed therein may be extrusion-or injection-molded to shaped articles, including films. The film so prepared is, however, poor in transparency and does not retain desirable properties such as high elongation at break inherently possessed by the polyesters.
Japanese Patent Laid-open Publication No. 54-46727 discloses a process for preparing a film in which a wholly aromatic polyester disclosed therein is melt-extruded through a slit to a film, followed by biaxial drawing of the film. Again, the film so prepared is poor in transparency.
Japanese Patent Laid-open Publication No. 58-104924 discloses that aromatic copolyesters having star-like branches disclosed therein may be cast to a film. But it makes no specific mention of any films, Further, the polyesters disclosed therein are quite specific and distinct from polyesters envisaged herein.